Anodic synthesis of benzaldehydes—I. Voltammetry of the anodic oxidation of toluenes in non-aqueous solutions

Abstract

The electrode kinetic investigation of the three consecutive steps of anodic toluene oxidation in methanol, namely the two-electron oxidation of toluene to benzylalcohol methylether, the two-electron oxidation of benzylmethylether to benzaldehyde dimethylacetal and the multielectron oxidation of benzaldehyde dimethylacetal to a mixture of unknown products was performed, using voltammetry at rotating and stationary disc electrodes and the rotating ring-disc electrodes in methanol, ethanol and acetonitrile as solvents. Investigated were toluene, benzylalcohol and benzaldehyde as references and para-substituted toluenes, benzylalcohols and benzaldehydes. Substituents with (+)I-effect activate the toleune towards anodic attack, substituents with (−)I-effect deactivate the aromatic system. A detailed electrode kinetic investigation is possible for p-methoxytoluene, the respective benzylalcohol and benzaldehyde. The current-voltage curve is almost exclusively mass transfer controlled. The chemical reaction following the first charge transfer is a first order reaction and is likely to be the proton dissociation of the toluene radical cation. It is heterogeneously catalysed as the choice of the anode material influences the half wave potential. In acetonitrile, as a more acidic solvent, dimerization seems to become important. A full description of the current-voltage curves by appropriate modelling is impossible as experimental inaccuracies do not allow for proper determination of the kinetic parameters. The proton dissociation rate of p-methoxytoluene radical cations can be estimated to be faster than 10 5s −1.